Contamination control in the manufacture of semiconductors is ever increasingly important. Particulate contamination causes more than half of the yearly losses in volume semiconductor manufacturing. A substantial amount of this loss is due to chemicals such as solvents, acids, bases and process gases that come into contact with the wafers. The contaminant concentration in such fluids is typically more than three orders of magnitude greater than that present in clean room air and six orders of magnitude greater than that present in the next generation of clean rooms.
The prior art is replete with instruments and methods for detecting particles by measuring scattered light. Traditionally, such light scattering is measured by determining the scattered light intensity from a particle or collection of particles. The forward direction is always excluded due to the presence of the incident beam. It is known that the relationship between a forward scattered field from a small particle and the focused incident beam is such that light scattered by the particle causes both a phase shift and an attenuation of the incident beam. The latter of these is called the optical extinction effect.
In U.S. Pat. Application Ser. No. 07/184,639 entitled "Particulate Inspection of Fluids", by Batchelder et al. and assigned to the same assignee as this application, the phase shift experienced by an incident beam is employed to differentiate between bubbles and particles in a fluid. In that patent application, as well as in an article by the inventors which appeared in Applied Physics Letters, Vol. 55, No. 3, July 1989, pp. 215-217, it is shown that a small dielectric particle in a focused monochromatic light beam, produces a scattered wave in phase quadrature with the far-field incident beam, thereby causing a phase shift in the beam. The forward scattered field is detected using a bright field interferometer which measures the phase shift in one beam relative to another. As a particle enters the first beam, it causes a phase shift in that beam relative to the second, with the resulting signal passing through zero at a point between the two beams and then changing sign as the particle enters the second beam. Elliptical polarization results from the induced phase shift. The phase shift is detected by subtracting the optical energy oriented along the minor axis of the ellipse from the optical energy oriented along the ellipse's major axis.
In addition to contamination detection in fluids, it is important to detect particulate contamination of semiconductor surfaces. Various systems have been proposed for surface examination. An article by See et al. entitled "Scanning Differential Optical Profilometer for Simultaneous Measurement of Amplitude and Phase Variation", Applied Physics Letters, Vol. 53, No. 1, July 1988, pp. 10-12 describes a scanning optical profilometer which measures the differential phase/amplitude variations of light reflected off an object surface. The phase and amplitude of the reflected signals enable measurements of film thickness, reflectivity variations and surface flatness. The See et al. system employs a Bragg cell for interrogating the surface with two separate beams.
Heinrich et al. in "A Non-Invasive Optical Probe For Detecting Electrical Signals and Silicon IC's", Review of Progress in Quantitative NDE; edited by D. Thompson et al., Plenum Press, Vol. 7B, 1988, pp. 1161-1166, describe an optical probe system for detecting electrical signals in silicon integrated circuits. Carriers within the circuit perturb the index of refraction of the material and enable a Nomarski interferometer to detect such perturbations. In essence, Heinrich et al. detect a phase change between two optical beams focused on a surface being interrogated. Again, elliptical polarization results from the reflection of those beams and is detected in a differential sensing circuit Neither See et al. or Heinrich et al. apply their systems to particle detection or characterization.
The prior art systems mentioned in the above co-pending application enable a particle to be differentiated from a gas bubble and, in addition, enable the size of the particle to be estimated. But, in order to determine where contamination is originating, it would be useful if a particle could be further classified to enable it to be identified as to its composition. Knowing its composition will enable rapid identification of the source of the contaminant and its elimination.
Accordingly, it is an object of this invention to provide a system which classifies particles by a physical characteristic thereof.
It is still another object of this invention to provide a system for classifying particles in accordance with their complex refractive index.
It is still a further object of this invention to provide an improved system for classifying small particles in both fluid and solid environments.